This invention generally relates to blow molding methods for producing heat set plastic containers. More specifically, this invention relates to blow molding methods for producing biaxially oriented plastic containers with high crystallinity bases.
Recently, manufacturers of polyethylene terephthalate (PET) containers have begun to supply plastic containers for commodities that were previously packaged in glass containers. The manufacturers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable, and manufacturable in large quantities. Manufacturers currently supply PET containers for various liquid commodities, such as juices. They also desire to supply PET containers for solid commodities, such as pickles. Many solid commodities, however, require pasteurization or retort, which presents an enormous challenge for manufactures of PET containers.
Pasteurization and retort are both methods for sterilizing the contents of a container after it has been filled. Both processes include the heating of the contents of the container to a specified temperature, usually above 70xc2x0 C., for a duration of a specified length. Retort differs from pasteurization in that it also applies overpressure to the container. This overpressure is necessary because a hot water bath is often used and the overpressure keeps the water in liquid form above its boiling point temperature. These processes present technical challenges for manufactures of PET containers, since new pasteurizable and retortable PET containers for these commodities will have to perform above and beyond the current capabilities of conventional heat set containers. Quite simply, the PET containers of the current techniques in the art cannot be produced in an economical manner such that they maintain their material integrity during the thermal processing of pasteurization and retort.
PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to maintain its material integrity is related to the percentage of the PET container in crystalline form, also known as the xe2x80x9ccrystallinityxe2x80x9d of the PET container. Crystallinity is characterized as a volume fraction by the equation:   Crystallinity  =            ρ      -              ρ        a                            ρ        c            -              ρ        a            
where xcfx81 is the density of the PET material; xcfx81a is the density of pure amorphous PET material (1.333 g/cc); and xcfx81c is the density of pure crystalline material (1.455 g/cc). The crystallinity of a PET container can be increased by mechanical processing and by thermal processing.
Mechanical processing involves orienting the amorphous material to achieve strain hardening. This processing commonly involves stretching a PET container along a longitudinal axis and expanding the PET container along a transverse axis. The combination promotes biaxial orientation. Manufacturers of PET bottles currently use mechanical processing to produce PET bottles having roughly 20% crystallinity (average sidewall crystallinity).
Because of the current design of molding machines and of plastic containers, the base of a typical plastic container does not undergo significant mechanical or thermal processing and is prone to deformation imparted if subjected to a high temperature hot fill process. Prior methods have involved either premolding or post molding operations to crystallize the base, both occurring outside the mold and requiring several minutes to complete.
Thus, the manufacturers of PET containers desire an efficient and inexpensive method that produces PET containers having high crystallinity bases, which allow the PET containers to maintain their material integrity during subsequent shipment and use of the PET containers. It is therefore an object of this invention to provide such a container that overcomes the problems and disadvantages of the conventional techniques in the art. Another object of the invention is to provide a method for in mold crystallization of a base.
Accordingly, this invention provides for a blow molding method that produces PET containers having average base crystallinities of at least 30%, which allow the PET containers to maintain their material integrity during any subsequent pasteurization or retort processes, and during shipment and use of the PET containers.
At its broadest, the invention is a method for producing a heat set plastic container including providing a mold cavity having sidewall surfaces and base surfaces; providing a plastic preform within the mold cavity; expanding and stretching the preform into conformity with the sidewall surfaces and the base surfaces to form a plastic container having a sidewall and a base; and inducing crystallinity in the base of the plastic container by applying heat from the base surfaces of the mold cavity to the base of the plastic container and by applying heat from an interior portion of the plastic container to an interior surface of the base of the plastic container.
Further features and advantages of the invention will become apparent from the following discussion and accompanying drawings.